How does oxytocin control the brain's social reward circuit?

Although several studies have pointed to oxytocin, or the "love hormone," as an important factor in promoting sociability, the mechanisms behind this remain unknown. Researchers from Stanford University have now looked into how oxytocin regulates the social reward mechanism in the brain.

Oxytocin, which is sometimes referred to as the "love hormone," is a hormone and neuropeptide - or neurotransmitter, carrying information through the central nervous system - involved in sociability and sexual interaction. It also plays a role in facilitating biological processes related to childbirth, and bonding with the newborn baby.

Oxytocin is mainly produced in a brain region called the paraventricular nucleus, which is located in the hypothalamus. The hypothalamus is involved in various metabolic processes, including regulating body temperature, determining states of hunger and thirst, and some social behaviors, such as attachment.

Recently, much has been made of oxytocin's role in promoting social behaviors, especially with a view to harnessing its potential in managing conditions such as autism, which impairs social interaction.

Medical News Today, for instance, have recently covered a study suggesting that the hormone could improve sociability in some children with autism.

Now, a new study conducted by Dr. Robert Malenka and a team of researchers from the Stanford University School of Medicine in California now looks at the mechanism behind the social reward system of the brain, pinpointing oxytocin's role in this process.

"[D]eficits in social behavior [due to brain disorders such as autism] profoundly affect [...] quality of life," Dr. Malenka told MNT, "and thus it is critical to understand the underlying abnormalities in brain function that cause [them]."

He continued, saying, "The findings in this paper suggest that one factor contributing to social behavior deficits may be abnormal modulation of the brain's reward circuitry by oxytocin."

A paper detailing the researchers' findings has been published in the journal Science.

The brain's reward mechanism

In the brain, a region called the ventral tegmental area (VTA) communicates with the nucleus accumbens to regulate the system's reward response, which is a circuit telling us that certain activities - such as eating, drinking, and sex - are pleasurable.

Thus, they "encourage" us to keep performing them. This reward mechanism is, in part, what ensures that we thrive and keep on reproducing.

The nerve cells that make up the VTA secrete dopamine, another neurotransmitter, which regulates the sensation of pleasure. Dopamine released in the brain makes us feel good when we perform activities correlated with survival, but abnormal levels of this neurotransmitter have also been linked to addiction and substance abuse.

Dr. Malenka and team were interested in understanding why dopamine is sometimes released abnormally, causing undesirable effects, and what other factors are implicated in the complex reward response mechanism of the brain.

Oxytocin impacted dopamine neurons

Since the reward circuit of mice is similar to that of humans, the researchers used the rodent model to study the mechanism's intricacies in more detail.

In a previous study on mice conducted by Dr. Malenka and colleagues, it became apparent that oxytocin plays an important role in determining social reward responses alongside dopamine. However, it remained unclear exactly how oxytocin impacted the functioning of this circuit.

Now, the researchers reveal that the paraventricular nucleus releases oxytocin in the VTA, which is crucial for promoting prosocial behaviors. Conversely, when the release of oxytocin into the VTA is inhibited, social interaction is impaired.

Dr. Malenka and team noted that oxytocin released in the VTA stimulates a group of neurons called "dopamine neurons," which function via dopamine signaling. Moreover, while inhibiting the release of oxytocin in that region of the brain did impact sociability negatively, it did not stop the animals' taste for pleasure-inducing drugs such as cocaine.

This is possibly the first study to confirm the existence of this mechanism, showing that oxytocin directly affects dopamine neurons in the VTA.

Crucial 'to understand how oxytocin works'

Dr. Malenka told MNT that the study "use[d] a sophisticated collection of methods to provide a brain mechanism that explains why social interactions are often pleasant and rewarding."

But he also pointed out that the main limitation it faced was the reliance on the mouse model, which may lead to some inaccuracies.

"The limitations," he explained, "are that the work was done in mice and therefore, of course, we do not know whether the same mechanisms happen in the human brain (although we think it is likely). We also use very simple behavioral assays in the mice and have to make the assumption that the social interactions we studied were 'rewarding'."

In the future, added Dr. Malenka, "We need to figure out experiments to do in people that will allow us to test whether the same mechanisms occur in the human brain and contribute to the good feeling we have when we have pleasant social interactions with our friends."

Research on the mechanisms behind the reward circuit, and the role that oxytocin plays in them, is important - especially given this hormone's potential to substantially affect social behavior.

"[O]xytocin is being tested as a potential therapeutic agent in the treatment of individuals with autism and other brain disorders which exhibit social behavior deficits. It is therefore important to understand how oxytocin works in the brain to mediate its potential therapeutic effects."

Dr. Robert Malenka

Going forward, he hopes that his team may be able to delve even deeper into the mysteries of the social reward response in the brain.

"We need to learn more about the detailed molecular mechanisms by which oxytocin modulates reward circuitry and dopamine neuron activity," he concluded.